The present invention relates to a stable driving method for a display element comprising an organic thin-film light-emitting element.
Research on organic thin-film light-emitting elements has recently been conducted actively because of the need for low-drive voltages that emit light and enable light-emitting colors to be selected by applying various light-emitting materials (for example, see U.S. Pat. No. 3,530,325). Such research has been further encouraged by reports that a brightness of 1,000 Cd/m.sup.2 or higher could be achieved at a drive voltage of 10 V or less by using a laminated organic thin-film light-emitting element consisting of an anode, positive hole injection layer, light-emitting layer and cathode in order to improve light-emitting efficiency (for example, see U.S. Pat. No. 4,356,429).
FIG. 9 is a cross-sectional view showing a conventional organic thin-film light-emitting element. An anode 2 that is a transparent conductive film, a positive hole injection layer 3 that is an organic material, a light-emitting layer 4, an electron injection layer 5, and a cathode 6 that is a metallic material are formed on a transparent substrate 1 that acts a support. The cathode 6 also has a function for reflecting light from the light-emitting layer 4 to improve efficiency in outputting light from the substrate 1.
FIG. 10 is a cross-sectional view showing a different conventional organic thin-film light-emitting element. In this element, the light-emitting layer 4' also incorporates the function of the electron injection layer 5.
In the conventional organic thin-film light-emitting elements as described above, the resistance of the element increases as light is continuously emitted. Thus, if the element is continuously driven at a constant voltage, the current flowing through pixels will decrease and the emitted light will be significantly attenuated.
In addition, the degree of increase in the resistance of the element differs in each pixel, and if the element is continuously driven at a constant voltage, after-image will appear that reflects the degradation of each element, thereby causing the screen to get fixed.
When an attempt is made to execute gradation display required for high-quality display by using the brightness of the element, if a control is made through the supply voltage for the element, the small range of control voltage will cause the screen to become clearly fixed depending on the degree of the degradation of each pixel. This results in the degradation of display quality.
Referring to FIG. 3, which is a diagram showing the relationship between the current flowing through a pixel and the applied voltage, a characteristic line 11 shows a current voltage characteristic in the initial phase of driving, while a characteristic line 12 shows one after a specified period of time has passed. The relationship 11 between the applied voltage and current during the initial phase of driving is basically similar to an exponential function, and the current rises rapidly with the voltage. As the driving time increases, the characteristic line 12 varies as shown at 12. When the driving is continued at the initial voltage 13, the current decreases from an initial value 14 to a value 15. The organic thin-film light-emitting element emits light due to the recombination of injected charges. The efficiency decreases due to the degradation associated with the driving, but is basically proportional to the value of the current. Thus, the amount of emitted light decreases rapidly with the current. Consequently, as the general methods for driving an organic thin-film light-emitting element, driving is made while maintaining the voltage flowing through the overall element at a constant level. In this case, the element is driven by increasing the applied voltage 16 in order to maintain the initial value 14.
FIG. 4 is a diagram showing the relationship between the brightness and driving time of an organic thin-film light-emitting element. A characteristic line 10 shows the characteristic obtained when the element is driven at a constant current, whereas a characteristic line 9 shows the characteristic obtained when the element is driven at a constant voltage. As shown in FIG. 4, when the element is driven at a constant current, the efficiency decreases and the brightness gradually decreases over time. The degradation of the characteristic is significantly smaller than that when it is driven at a constant voltage.
If, for example, the element shown in FIG. 10 is used, the brightness will decrease by half in about 20 hours when the element is continuously driven at an initial brightness of 100 Cd/m.sup.2 and a constant voltage. On the contrary, when the element is driven in such a way that the voltage is compensated for so as to provide a constant current, the same element will not have brightness reduced by half until about 500 hours have passed. It is thus well known that driving the element at a constant current is effective in increasing the life expectancy of the organic thin-film light-emitting element.
The method of driving the element at a constant current, however, is effective in increasing the life expectancy of the element, but can not prevent the fixing of the screen caused by the non-uniform degradation of the pixels. In addition, this method fails to provide a high-quality display with gradations.
The invention has been made in view of these problems, and its object is to improve the constant-current driving method to provide a display-element driving method that is unlikely to cause the screen to get fixed and that can ensure high-quality display.